1. Technical Field
The description set forth herein generally relates to adhesive films for connecting conductive elements to other items. More particularly, the description relates to adhesive films that may be used to attach a conductive element such as a printed circuit board to another device such as an element of an electronic display.
2. Description of the Related Art
Display technologies based on encapsulation of electrophoretic particles, multichromal beads and liquid crystals have many potential applications in fields such as electronic paper and other digital document media. Examples of such electronic display devices include those available from Gyricon LLC of Ann Arbor Mich. and those described in, for example, U.S. Pat. Nos. 6,703,074; 5,389,945; 4,438,160; 4,261,653; 4,143,103; and 4,176,854, each of which is incorporated by reference herein in its entirety. In one such display device, as shown in FIG. 1, bichromal beads, cylinders, crystals or other bichromal or multichromal particles 10 are dispersed in an elastomeric sheet swollen by a fluid 12 and positioned atop a conductive substrate 14 such as a printed circuit board. The particles, fluid and substrate are covered with a transparent layer 16 such as glass or plastic and a transparent conductive material such as indium tin oxide (ITO) 18, and they are sealed to form a re-addressable display material in which the particles rotate in response to an electric or magnetic field that is applied to the conductive substrate.
The conductive substrate is often flexible, and it may include tens, hundreds or even thousands of very small, very closely spaced electrical terminals. The terminals are interconnected by a complex web of traces that provide conductivity between selected elements of the circuit board.
In the assembly of an electric display such as that shown in FIG. 1, it is known to use an anisotropic conductive adhesive, commonly known as a Z-axis adhesive, to connect the circuit board to the display. If the adhesive is conductive across the entire circuit board, it will interfere with the traces by providing additional paths of conductivity between the elements of the board. The anisotropic conductive adhesive provides electrical conductivity only in a direction perpendicular to the connected surfaces and not in a direction parallel to the surfaces. Thus, the anisotropic adhesive does not create undesired additional paths of conductivity between the elements of the circuit board. Anisotropic conductive adhesives typically include conductive particles dispersed throughout an insulating adhesive matrix, such as an epoxy or a polymer. Examples of anisotropic conductive adhesives are further described in U.S. Pat. Nos. 5,685,939 and 5,362,421, each of which is incorporated herein by reference in its entirety.
When used in a display device, the anisotropic film delivers a field from the circuit board to the image formation layer through the conductive particle chains that extend across the thickness of the film, thus electrically connecting the connected circuit board terminals and the image formation layer. A high concentration of conductive particles (e.g., 20%-30%) is currently necessary to provide sufficient electrical conductivity across the layer. However, if the concentration is too high, lateral particle contacts, and thus short circuits, can result between adjacent electrical terminals on the board. This can create a reliability problem in applications requiring a fine pitch. In addition, a high loading of conductive particles can increase the cost of manufacturing, since conductive particles can be expensive.
In addition, because of their random distribution, local variations of particle concentration can occur in an adhesive matrix later. An example of such a particle distribution in a conductive adhesive film 20 is illustrated in FIG. 2, wherein the conductive particles 24 are randomly distributed in the adhesive matrix 22. This distribution may cause non-uniform conductivity across the film. In extreme situations, this can even cause an open circuit in the perpendicular direction or a short circuit in the parallel direction if the distribution is highly varied.
Accordingly, I have found that a need exists for an improved anisotropic conductive adhesive, as well as improved methods of making displays and other electrical devices using an improved anisotropic conductive adhesive.